Since .net framework 4.0, the gacutil location has been changed and in a windows sdk folder.
However, on a SharePoint 2013 production box, the windows SDK is not installed.
Here is the Powershell come to rescue.
[System.Reflection.Assembly]::Load("System.EnterpriseServices, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a")
$publish = New-Object System.EnterpriseServices.Internal.Publish
$publish.GacInstall("...you dll path");
